1. Technical Field
The present invention relates to a catalyst for use in producing a carboxylic ester from an aldehyde, an alcohol and molecular oxygen, a method for producing the catalyst, and a method for the production of a carboxylic ester by using the catalyst. More particularly, the present invention is concerned with a novel catalyst which comprises a carrier having supported thereon palladium and lead in a specific atomic ratio, and which exhibits a maximum intensity peak at a diffraction angle (2.theta.) in a specific range in a powder X-ray diffraction pattern thereof. The present invention is also concerned with a method for producing the catalyst and a method for the continuous production of a carboxylic ester from an aldehyde, an alcohol and molecular oxygen by using the catalyst. In the production of a carboxylic ester using the catalyst of the present invention, the desired carboxylic ester can be obtained with high selectivity even when the reaction is conducted under high temperature and high aldehyde concentration conditions. Further, the catalyst of the present invention exhibits high mechanical strength and high corrosion resistance. In the continuous method for producing a carboxylic ester using the catalyst of the present invention, the desired carboxylic ester can be produced with high efficiency and in high yield for a prolonged period of time.
2. Prior Art
Methods for producing methyl methacrylate or methyl acrylate, which is commercially useful, have already been practiced on a commercial scale. For example, a method for producing methyl methacrylate has been commercially practiced in which methacrolein is oxidized with molecular oxygen to produce methacrylic acid and then, the methacrylic acid produced is reacted with methanol to produce methyl methacrylate. As a result of various attempts for improving catalysts over many years, the yield of methacrylic acid in the above-mentioned step of oxidizing methacrolein has been improved to some extent, namely to a value in the range of from 80% to 85%, which is, however, still unsatisfactory from the commercial viewpoint. Therefore, with respect to the yield of methacrylic acid in the above step, a further improvement has been desired. Further, heteropolyacid catalysts, which are conventionally used in the above reaction for oxidizing methacrolein to produce methacrylic acid, have problems in that they have poor thermal stability, so that the heteropolyacid catalysts are gradually decomposed at the reaction temperatures. It has been reported that improvements have been made in the thermal stability of heteropolyacids. However, it is generally considered that even such improved catalysts are still unsatisfactory in catalytic life when used in commercial practice for the production of methacrylic acid from methacrolein.
On the other hand, a new process for producing methyl methacrylate or methyl acrylate has been attracting attention, in which methacrolein or acrolein is reacted with methanol and molecular oxygen to there-by produce methyl methacrylate or methyl acrylate by a single-step process {hereinafter, methacrolein and acrolein are frequently collectively referred to as "(meth)acrolein", and methyl methacrylate and methyl acrylate are frequently collectively referred to as "methyl (meth)acrylate"}. In this process of reacting (meth)acrolein with methanol and molecular oxygen, it is essential to use a catalyst containing palladium.
Conventionally, the above-mentioned single-step process has a problem in that an aldehyde decomposition reaction occurs as a side reaction to thereby by-produce a hydrocarbon and carbon dioxide gas, so that the yield of a carboxylic ester as the desired product is low. This method is also disadvantageous in that the alcohol undergoes oxidation as a side reaction to by-produce an aldehyde which is different from the aldehyde used as a starting material, and the by-produced aldehyde causes formation of an undesired carboxylic ester which is different from the desired carboxylic ester, so that the selectivity based on the alcohol is also poor (for example, when the alcohol is methanol, methyl formate is by-produced as an undesired carboxylic ester, and when the alcohol is ethanol, methyl acetate is by-produced as an undesired carboxylic ester). Further, there is also a disadvantage that the conventional catalyst cannot maintain its activity for a long period of time. Especially when a commercially valuable process for producing a carboxylic ester from an .alpha..multidot..beta.-unsaturated aldehyde {such as (meth)acrolein} as a starting material, is practiced, a large amount of decomposition products is produced, such as carbon dioxide gas and an olefin (which is propylene when methacrolein is used as a starting material). Thus, this process has not been successfully practiced.
In Examined Japanese Patent Application Publication (Japanese Kokoku) Nos. 57-035856, 57-035857, and 57-035859, the present inventors proposed a catalyst containing palladium and lead, and showed that, in the production of a carboxylic ester using this catalyst, the selectivity for methyl (meth)acrylate based on (meth)acrolein can be largely improved to a level as high as more than 90%. However, with this catalyst, the selectivity for methyl (meth)acrylate is low when the reaction temperature is 50.degree. C. or more, so that it is difficult to produce methyl (meth)acrylate with high economical efficiency.
Subsequently, in Examined Japanese Patent Application Publication (Japanese Kokoku) No. 62-007902, the present inventors proposed a catalyst comprising an intermetallic compound in which palladium and lead are bonded to each other in a simple integral ratio. In this Japanese patent document, the present inventors showed that, in the production of a carboxylic ester using this catalyst, the decomposition of (meth)acrolein is almost completely suppressed, and the catalyst activity can be maintained for a prolonged period of time.
The new single-step process using these new types of catalysts has also an advantage in that the process can be performed in one step, as compared to the conventional two-step process in which a desired carboxylic ester is produced via (meth)acrylic acid and in which the yield of the desired carboxylic ester and the life of the catalyst are still unsatisfactory. Therefore, it has been desired that the new single-step process be commercialized as a new method for producing commercially useful raw materials for various valuable polymers.
However, when the production of, for example, methyl methacrylate (MMA) is conducted, using the above-mentioned new catalysts, under economically advantageous reaction conditions necessary for commercially practicing the new method, i.e., under reaction conditions such that the reaction temperature is as high as 60.degree. C. or more and the methacrolein concentration of the reaction system is as high as 20% or more, not only does the selectivity for MMA become low, but also the by-production of methyl formate due to the oxidation of methanol is sharply increased. For example, Examined Japanese Patent Application Publication (Japanese Kokoku) No. 62-007902 shows that not only can an MMA selectivity exceeding 90% be achieved, but also the by-production of methyl formate is suppressed to a level as low as 0.03 to 0.06 mole/mole of MMA. However, these favorable results can be obtained only when the reaction is conducted under moderate reaction conditions such that the methacrolein concentration of the reaction system is as low as 10% or less and the reaction temperature is as low as 40 to 60.degree. C. When the reaction is conducted under such moderate reaction conditions, the MMA concentration of the resultant reaction mixture is low and, hence, the recycling of a large amount of unreacted methanol inevitably becomes necessary, so that a large amount of vapor becomes necessary for recycling the unreacted methanol. In addition, in this case, since the productivity of the method is low, it is necessary to use a reactor having a large volume. Therefore, this proposal is economically disadvantageous. For obviating the above economical disadvantages of the reaction, it is desired to increase the methacrolein concentration of the reaction system as much as possible and elevate the reaction temperature to a level as high as possible. Examined Japanese Patent Application Publication (Japanese Kokoku) No. 5-069813 shows a reaction example in which the methacrolein concentration of the reaction system is 20% and the reaction temperature is 80.degree. C. However, in this reaction conducted under such high methacrolein concentration and high reaction temperature conditions, it is impossible to obtain a MMA selectivity, based on methacrolein, as high as more than 90%. Further, in this reaction conducted under the above severe reaction conditions, a marked by-production of methyl formate occurs in an amount as large as 0.0923 mole/mole of MMA, which is about 2 times the amount of methyl formate by-produced in the reaction conducted under the above-mentioned moderate reaction conditions. Moreover, when this reaction is conducted under more severe reaction conditions such that the methacrolein concentration is increased up to 30%, a decomposition reaction of methacrolein is likely to occur, so that the MMA selectivity based on methacrolein becomes further lowered.
Palladium is a precious metal and expensive and, hence, when palladium is used as a component for a catalyst, palladium is usually used in the form of a composition comprising a carrier having palladium which is dispersedly supported on the carrier. The selection of an appropriate carrier is extremely important. In Unexamined Japanese Patent Application Laid-Open Specification (Japanese Kokai) Nos. 57-35856, 57-35857, 57-35858, 57-35859 and 57-35860, activated carbon, silica, alumina, calcium carbonate and the like are mentioned as examples of carriers in connection with the method for producing a carboxylic ester by reacting an aldehyde, an alcohol and oxygen in the presence of a palladium-containing catalyst. In order to examine whether or not catalysts employing these carriers can be used for a commercial production process, the present inventors made intensive studies on palladium catalysts employing these carriers. As a result, they found that these catalysts are not completely satisfactory with respect to the catalyst life under the reaction conditions. Specifically, it was found that when the reaction is conducted using an activated carbon-carried palladium catalyst in a slurry form in an agitation type reactor or a bubble column reactor (which is frequently employed in a commercial production process), the activated carbon carrier is unsatisfactory in mechanical strength. Further, it was found that although an alumina carrier has high mechanical strength, an alumina carrier has an inherent disadvantage in that it is corroded with an acidic substance, such as methacrylic acid or acrylic acid, which acidic substance is a typical by-product of the reaction, so that the expensive palladium supported on the alumina carrier is likely to come-off from the carrier. It was also found that a calcium carbonate carrier is more likely to be corroded with an acidic substance than an alumina carrier and, hence, is unsuitable for use in a commercial production process. With respect to a silica carrier, an unfavorable phenomenon was observed such that the silica carrier is gradually corroded with water present in the reaction system, causing the silica to be dissolved out in the reaction system. Thus, this suggests that a silica carrier cannot be stably used for a long period of time. Further, it was found that a silica carrier has poor mechanical strength, as compared to an alumina carrier. As apparent from the above, a catalyst carrier which has not only high mechanical strength and high physical stability, but also high chemical stability such that it is not susceptible to corrosion with water, an acidic substance and an alkaline substance, has not yet been proposed.
It has been reported that, in an attempt to improve the mechanical strength and hydrolysis resistance of a silica carrier, a silica gel is modified by, for example, changing the production method of a silica gel or subjecting a silica gel to sintering. However, there is no report that both of the mechanical strength and hydrolysis resistance of a silica carrier can be improved simultaneously without impairing the desired properties of the catalyst using the silica carrier. For example, it is known that quartz, which is one type of silica, is hard and has both high mechanical strength and high hydrolysis resistance. However, quartz has a small specific surface area (1 m.sup.2 /g or less). Therefore, quartz cannot support a metal thereon in the form of well dispersed fine particles, so that a catalyst using quartz as a carrier has extremely low reaction activity per unit weight. For this reason, it is difficult to use quartz as a carrier in a catalyst.
As described hereinabove, a material for a carrier to be used in a catalyst is required to have properties such that it not only maintains a large specific surface area for a satisfactorily long period of time, but also has both high mechanical strength and high corrosion resistance to a liquid corrosive substance inherent in the reaction for producing a carboxylic ester. Conventionally, a carrier satisfying all of these requirements has not been known. Therefore, for achieving the economic improvement of the reaction, it has been desired to develop a catalyst which not only can be used for producing a desired carboxylic ester (e.g., MMA) with a selectivity as high as more than 90% even when the reaction is conducted under high temperature and high aldehyde concentration conditions, while suppressing the production of by-products (such as methyl formate), but also has both high mechanical strength and high corrosion resistance.